1. Field of the Invention
The invention relates to magnetically triggered elapsed time indicators for use on moving vehicles, and more specifically to such indicators using validated output signals for initiating and terminating the measurement of elapsed time. Still more specific features relate to apparatus for determining the average speed of a boat moving through a course marked by magnetic buoys.
2. Description of the Prior Art
It is desirable in many competitive events to know on-board a moving vehicle the average speed of travel between fixed points. In water skiing events, such as slalom and jumping, for example, a towing boat is required to travel at speeds within relatively close tolerances between two rows of buoys that define the course. Elapsed times or average boat speeds are measured electronically to confirm the run is within the required tolerances and to validate compliance with the rules of competition. Similar requirements exist in motor racing, where the vehicle may travel in laps that return to a fixed point. Lap speeds, or the elapsed time taken to travel identified sections of the course, can provide competitive advantages, particularly when immediately available to the driver.
Timing devices for water skiing have evolved with the sport from simple stop watches to automated systems. It is common to mount magnets on buoys that mark the course, and to sense the resulting magnetic field with an electrical coil carried by the tow boat. The coil produces a signal when it moves in the field past each magnetic buoy. The signal from the first buoy starts a timer when the boat enters the course. The last buoy produces a similar signal at the end of the course and stops the timer to provide a measurement of the elapsed time for the boat to travel between the first and last buoys. Since the distance between the buoys is known, the elapsed time represents the average speed, which is compared to a permitted range to validate the run.
U.S. Pat. No. 4,392,122, issued to Redvers Hocken on Jul. 5, 1983, includes a history and description of existing practice. According to the Hocken disclosure, three of the buoys marking a slalom course are provided with magnets at entrance, exit and intermediate points along the course. Two timers are started when the boat passes the first buoy. One is stopped at the intermediate buoy and the other at the exit buoy. The second timer then provides an indication of average boat speed over the full length of the course, while the first timer permits mid-course corrections based on the average speed up to the intermediate buoy.
Hocken recognizes problems caused by extraneous magnetic fields emanating from the boats ignition and other electrical sources. The sensor carried by the boat preferably has directional properties aligned to maximize the influence of fields originating from the buoy-mounted magnets and to minimize the influence of extraneous fields. Directional properties are achieved by using an elongate coil including a magnetically permeable core extending from the ends of the coil.